From: Shaohua Li shli@fb.com
Patch series "userfaultfd: write protection support", v6.
Overview ========
The uffd-wp work was initialized by Shaohua Li [1], and later continued by Andrea [2]. This series is based upon Andrea's latest userfaultfd tree, and it is a continuous works from both Shaohua and Andrea. Many of the follow up ideas come from Andrea too.
Besides the old MISSING register mode of userfaultfd, the new uffd-wp support provides another alternative register mode called UFFDIO_REGISTER_MODE_WP that can be used to listen to not only missing page faults but also write protection page faults, or even they can be registered together. At the same time, the new feature also provides a new userfaultfd ioctl called UFFDIO_WRITEPROTECT which allows the userspace to write protect a range or memory or fixup write permission of faulted pages.
Please refer to the document patch "userfaultfd: wp: UFFDIO_REGISTER_MODE_WP documentation update" for more information on the new interface and what it can do.
The major workflow of an uffd-wp program should be:
1. Register a memory region with WP mode using UFFDIO_REGISTER_MODE_WP
2. Write protect part of the whole registered region using UFFDIO_WRITEPROTECT, passing in UFFDIO_WRITEPROTECT_MODE_WP to show that we want to write protect the range.
3. Start a working thread that modifies the protected pages, meanwhile listening to UFFD messages.
4. When a write is detected upon the protected range, page fault happens, a UFFD message will be generated and reported to the page fault handling thread
5. The page fault handler thread resolves the page fault using the new UFFDIO_WRITEPROTECT ioctl, but this time passing in !UFFDIO_WRITEPROTECT_MODE_WP instead showing that we want to recover the write permission. Before this operation, the fault handler thread can do anything it wants, e.g., dumps the page to a persistent storage.
6. The worker thread will continue running with the correctly applied write permission from step 5.
Currently there are already two projects that are based on this new userfaultfd feature.
QEMU Live Snapshot: The project provides a way to allow the QEMU hypervisor to take snapshot of VMs without stopping the VM [3].
LLNL umap library: The project provides a mmap-like interface and "allow to have an application specific buffer of pages cached from a large file, i.e. out-of-core execution using memory map" [4][5].
Before posting the patchset, this series was smoke tested against QEMU live snapshot and the LLNL umap library (by doing parallel quicksort using 128 sorting threads + 80 uffd servicing threads). My sincere thanks to Marty Mcfadden and Denis Plotnikov for the help along the way.
TODO ====
- hugetlbfs/shmem support - performance - more architectures - cooperate with mprotect()-allowed processes (???) - ...
References ==========
[1] https://lwn.net/Articles/666187/ [2] https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/log/?h=userfau... [3] https://github.com/denis-plotnikov/qemu/commits/background-snapshot-kvm [4] https://github.com/LLNL/umap [5] https://llnl-umap.readthedocs.io/en/develop/ [6] https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/commit/?h=user... [7] https://lkml.org/lkml/2018/11/21/370 [8] https://lkml.org/lkml/2018/12/30/64
This patch (of 19):
Add helper for writeprotect check. Will use it later.
Signed-off-by: Shaohua Li shli@fb.com Signed-off-by: Andrea Arcangeli aarcange@redhat.com Signed-off-by: Peter Xu peterx@redhat.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Reviewed-by: Jerome Glisse jglisse@redhat.com Reviewed-by: Mike Rapoport rppt@linux.vnet.ibm.com Cc: Rik van Riel riel@redhat.com Cc: Kirill A. Shutemov kirill@shutemov.name Cc: Mel Gorman mgorman@suse.de Cc: Hugh Dickins hughd@google.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Bobby Powers bobbypowers@gmail.com Cc: Brian Geffon bgeffon@google.com Cc: David Hildenbrand david@redhat.com Cc: Denis Plotnikov dplotnikov@virtuozzo.com Cc: "Dr . David Alan Gilbert" dgilbert@redhat.com Cc: Martin Cracauer cracauer@cons.org Cc: Marty McFadden mcfadden8@llnl.gov Cc: Maya Gokhale gokhale2@llnl.gov Cc: Mike Kravetz mike.kravetz@oracle.com Cc: Pavel Emelyanov xemul@openvz.org Link: http://lkml.kernel.org/r/20200220163112.11409-2-peterx@redhat.com Signed-off-by: Linus Torvalds torvalds@linux-foundation.org --- include/linux/userfaultfd_k.h | 10 ++++++++++ 1 file changed, 10 insertions(+)
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h index 37c9eba75c98..38f748e7186e 100644 --- a/include/linux/userfaultfd_k.h +++ b/include/linux/userfaultfd_k.h @@ -50,6 +50,11 @@ static inline bool userfaultfd_missing(struct vm_area_struct *vma) return vma->vm_flags & VM_UFFD_MISSING; }
+static inline bool userfaultfd_wp(struct vm_area_struct *vma) +{ + return vma->vm_flags & VM_UFFD_WP; +} + static inline bool userfaultfd_armed(struct vm_area_struct *vma) { return vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP); @@ -94,6 +99,11 @@ static inline bool userfaultfd_missing(struct vm_area_struct *vma) return false; }
+static inline bool userfaultfd_wp(struct vm_area_struct *vma) +{ + return false; +} + static inline bool userfaultfd_armed(struct vm_area_struct *vma) { return false;